A transcatheter prosthetic valve that can be delivered endovascularly via a catheter can help to minimize patient trauma as compared with an open-heart, surgical procedure. Open heart surgery involves extensive trauma to the patient, with attendant morbidity and extended recovery. A valve delivered to the recipient site via a catheter avoids the trauma of open heart surgery and may be performed on patients too ill or feeble to survive the open heart surgery.
Transcatheter valve implantation with currently available transcatheter valves and associated delivery catheters, together referred herein as delivery systems, present several procedural-related complications. Trauma to the peripheral vasculature as well as dissection of the ascending and descending aorta has been observed. This trauma is associated, in part, with the relatively large diameter of the delivery systems. Minimizing such trauma can be facilitated by minimizing the diameter of the delivery system which is determined, in part, by the profile of the valve on the associated delivery catheter.
Reducing the profile of the prosthetic heart valve on the delivery catheter is technically challenging. For example, a 23 mm diameter aortic prosthetic valve might have to be advanced through 10 mm diameter vasculature to reach the deployment site. This requires that the valve be compressed to a smaller diameter upon the delivery catheter such that it and the delivery catheter present a diameter somewhat smaller than 10 mm.
The profile of the valve is dependent, in part, on the valve components. Some transcatheter valve devices comprise a valve having flexible leaflets mounted inside a tubular metal frame. The metal frame may be self expanding or balloon-expanded from a pre-deployed compressed diameter to the deployed functional diameter. The diameter of the delivery system is dependent, in part, on the resulting thickness of the compressed valve leaflets within the frame as it is mounted on the delivery catheter.
The transcatheter valve must be capable of being securely coupled to the tissue orifice of the implantation site after endovascular placement so as to avoid, for example, dislodgement or migration of the valve after placement. The coupling of the valve to the implantation site is commonly facilitated by relatively high hoop strength of the frame placed in urging engagement with the tissue orifice.
Bioprosthetic valves have been developed that attempt to mimic the function and performance of a native valve. Flexible leaflets are fabricated from biological tissue such as bovine pericardium. In some valve designs the biological tissue is sewn onto a relatively rigid frame that supports the leaflets and provides dimensional stability when implanted. Although bioprosthetic valves can provide excellent hemodynamic and biomechanical performance in the short term, they are prone to calcification and cusp tears, among other failure modes, requiring reoperation and replacement.
Attempts have been made to use synthetic materials, such as polyurethane, among others, as a substitute for the biological tissue, to provide a more durable flexible leaflet prosthetic valve, herein referred to as a synthetic leaflet valve (SLV). However, synthetic leaflet valves have not become a valid valve replacement option since they suffer premature failure, due to, among other things, suboptimal design and lack of a durable synthetic material.
A number of fabrication techniques have been used to couple the leaflets to a frame, including sewing individual leaflets to the frame (biological and synthetic), and for synthetic leaflets only, injection molding and dip coating a polymer onto the frame. In each case, the resulting leaflet is supported on the frame and defines a flap having a mounting edge where the leaflet is coupled to the frame and a free edge that allows the flap to move. The flap moves under the influence of fluid pressure. In operation, the leaflets open when the upstream fluid pressure exceeds the downstream fluid pressure and close when the downstream fluid pressure exceeds the upstream fluid pressure. The free edges of the leaflets coapt under the influence of downstream fluid pressure closing the valve to prevent downstream blood from flowing retrograde through the valve.
Valve durability under the repetitive loads of the leaflets opening and closing is dependent, in part, on the load distribution between the leaflet and the frame. Further, substantial load is encountered on the leaflet when in the closed position. Mechanical failure of the leaflet can arise, for example, at the mounting edge, where the flexible leaflet is supported by the relatively rigid frame. The repetitive loads of leaflet opening and closing leads to material failure by fatigue, creep or other mechanism, depending in part on the leaflet material. Mechanical failure at the mounting edge is especially prevalent with synthetic leaflets.
There exists a need for a durable transcatheter prosthetic valve that is compressible to a small diameter and capable of being delivered endovascularly.